Heterogeneous catalysts have advantages over homogenous catalysts including the ability to recycle and reuse the catalyst, and the absence of the active catalyst species in the reaction products. Usually, a heterogeneous catalyst consists of catalytically active particles deposited on support particles that are catalytically inactive (e.g., silica, alumina, etc.). In a few cases, the support particles are themselves catalytically active. The catalytically active particles are deposited onto the support by techniques such as impregnation of the catalytic particles, adsorption of catalyst species from a solution, homogeneous deposition precipitation onto a support, and chemical vapor deposition. The catalytic activity of the heterogeneous catalysts generally depends upon factors such as specific surface area, pore structure of the support particles, and the size of the active particles on the support. While these factors may be adjusted to try and increase catalytic activity, the catalytic activity of heterogeneous catalysts is usually lower than that of homogeneous catalysts, on a weight basis.
One attempt to increase catalyst activity has involved reducing the size of the active particles to nanoscale dimensions. Due to this, supported nano Pt catalysts in particular have gained tremendous attraction. The nano size of the Pt metal increases intrinsic catalytic activity through an increase in the surface to volume ratio and the specific surface area. (See U.S. Pat. No. 7,053,021.) Many of the existing methods for synthesizing supported nanoparticulate Pt catalysts, however, are the same as those used for synthesizing normal supported catalysts, i.e., impregnation, adsorption, homogeneous deposition, and chemical vapor deposition. In all of these, only physical interactions (Van der Waals forces, e.g.) exist between the catalytically active particles and the support. The physical interactions may not be strong enough to withstand the shear conditions in stirred or fixed bed reactors, especially for catalytically active particles of nano-size. Such catalysts are vulnerable to loss of the catalytically active particles to the reaction medium. This could significantly limit their recycle/reuse potential.
U.S. Patent Publication No. 2004/0087441A1 describes PtRu nanoparticles directly applied to the support by reducing precursor metal salts and depositing the nanoparticles on the support during synthesis. It may be noted that in such a catalyst the attractive forces between the active nanoparticles and the support are physical forces, and this may not provide sufficiently strong binding between the two. Such catalysts are likely to be vulnerable to nanoparticle loss in the high shear conditions of fixed bed and stirred reactors.
Other attempts to improve catalytic activity have involved adjusting the crystal structure, softness, etc., of the nanoparticles. For example, U.S. Pat. No. 6,177,585 describes the use of bi-metallic catalysts, although the catalyst particles are not in the nano size range.
One issue in prior catalyst systems is stabilization of the particles because the nanoparticles tend to aggregate in the absence of stabilizing agents. Traditionally, nanoparticle stabilization is achieved by using surfactants. Stabilization provided by surfactants is considered to be of an electrostatic nature. Steric stabilization of particles may be accomplished by encapsulating the nanoparticles in a crosslinked siloxane polymer as described in B.P.S. Chauhan et. al, Journal of Organometallic Chemistry, 686(1-2), p. 24-31, 2003. Such nanoparticles are reported to be catalytically active, capable of being separated from the product liquid by ultracentrifugation, and reused. However, ultracentrifugation is not a cost-effective method of separating the nanoparticles for industrial scale processes.